Mammalian cell growth, differentiation, and migration are directed by hormones and specific protein ligands, often termed cytokines. In particular, cells comprising the neuro-endocrine, hematopoietic and the immune/inflammatory systems are known to be governed by cytokines. Cytokines, like other ligands, interact with cells by means of specific receptors, usually expressed on the cell surface.
A fundamental problem confronting biomedical scientists is to discern how signals are transduced through ligand receptors and how these signals determine the response of the cell. Many ligands influence their target cells by stimulating the expression of specific genes. However, the genes signaled by most cytokines remain largely unknown owing to the complexity of cellular biochemistry. Moreover, the gene products that are vital for performing different cellular processes are often only expressed transiently, and/or in very low concentrations so that they are difficult to detect, isolate and characterize.
Interleukin-2 (IL-2) is a cytokine that is critical for the immune system: it directs the proliferation and differentiation of T lymphocytes (T-cells), B lymphocytes (B-cells), and natural killer (NK) cells. Just how IL-2 signals these cellular events in the various types of target cells remains unknown. A few genes have been identified that are expressed as a result of IL-2 stimulation of T cells. These include the cellular proto-oncogenes c-fos, c-myb, c-myc, pim-1, and c-raf-1. However, exactly how many and what other genes are expressed as a result of IL-2/IL-2 receptor interaction remains unknown.
Since the discovery of DNA cloning, methods have become available to isolate specific genes expressed by cells. However, it has been difficult to devise new methods to isolate and clone all or most of the genes expressed by a cell activated by a given ligand, a task that must be done before one can understand how the ligand directs the cell to perform specific functions. In addition, methods of identifying a particular gene or genes stimulated early on after ligand receptor activation have not been easily forthcoming as the number of genes stimulated by receptor activation from which a particular gene must be selected is usually quite large.
Therefore, what is needed are methods to select and enrich only for those genes stimulated by a given ligand. Ideally, these methods should detect those genes expressed in low concentrations, as well as those expressed at high concentrations.